Epitaxy of high tensile silicon alloy for tensile strain applications

ABSTRACT

Embodiments of the present invention generally relate to methods for forming silicon epitaxial layers on semiconductor devices. The methods include forming a silicon epitaxial layer on a substrate at increased pressure and reduced temperature. The silicon epitaxial layer has a phosphorus concentration of about 1×10 21  atoms per cubic centimeter or greater, and is formed without the addition of carbon. A phosphorus concentration of about 1×10 21  atoms per cubic centimeter or greater increases the tensile strain of the deposited layer, and thus, improves channel mobility. Since the epitaxial layer is substantially free of carbon, the epitaxial layer does not suffer from film formation and quality issues commonly associated with carbon-containing epitaxial layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/440,627, filed Feb. 8, 2011, which is herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to the field ofsemiconductor manufacturing processes and devices, more particularly, tomethods of depositing silicon-containing films for forming semiconductordevices.

2. Description of the Related Art

Size reduction of metal-oxide-semiconductor field-effect transistors(MOSFET) has enabled the continued improvement in speed performance,density, and cost per unit function of integrated circuits. One way toimprove transistor performance is through application of stress to thetransistor channel region. Stress distorts (e.g., strains) thesemiconductor crystal lattice, and the distortion, in turn, affects theband alignment and charge transport properties of the semiconductor. Bycontrolling the magnitude of stress in a finished device, manufacturerscan increase carrier mobility and improve device performance. There areseveral existing approaches of introducing stress into the transistorchannel region.

One such approach of introducing stress into the transistor channelregion is to incorporate carbon into the region during the formation ofthe region. The carbon present in the region affects the semiconductorcrystal lattice and thereby induces stress. However, the quality ofepitaxially-deposited films decreases as carbon concentration within thefilm increases. Thus, there is a limit to the amount of tensile stresswhich can be induced before film quality becomes unacceptable.

Generally, carbon concentrations above about 1 atomic percent seriouslyreduce film quality and increase the probability of film growth issues.For example, film growth issues such as undesired polycrystalline oramorphous silicon growth, instead of epitaxial growth, may occur due tothe presence of carbon concentrations greater than 1 atomic percent.Therefore, the benefits that can be gained by increasing the tensilestress of a film through carbon incorporation are limited to filmshaving carbon concentrations of 1 atomic percent or less. Moreover, evenfilms which contain less than 1 atomic percent carbon still experiencesome film quality issues.

Therefore, there is a need for producing a high tensile stress epitaxialfilm which is substantially free of carbon.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to methods forforming silicon epitaxial layers on semiconductor devices. The methodsinclude forming a silicon epitaxial layer on a substrate at increasedpressure and reduced temperature. The silicon epitaxial layer has aphosphorus concentration of about 1×10²¹ atoms per cubic centimeter orgreater, and is formed without the addition of carbon. A phosphorusconcentration of about 1×10²¹ atoms per cubic centimeter or greaterincreases the tensile strain of the deposited layer, and thus, improveschannel mobility. Since the epitaxial layer is substantially free ofcarbon, the epitaxial layer does not suffer from film formation andquality issues commonly associated with carbon-containing epitaxiallayers.

In one embodiment, a method of forming a film on a substrate comprisespositioning a substrate within a processing chamber, and heating thesubstrate to a temperature within a range from about 550 degrees Celsiusto about 700 degrees Celsius. One or more process gases are thenintroduced into the processing chamber. The one or more process gasescomprise a silicon source and a phosphorus source. A substantiallycarbon-free silicon epitaxial layer is then deposited on the substrate.The substantially carbon-free silicon epitaxial layer has a phosphorusconcentration of about 1×10²¹ atoms per cubic centimeter or greater. Thesubstantially carbon-free silicon epitaxial layer is deposited at achamber pressure of about 300 Torr or greater.

In another embodiment, a method of forming a film on a substratecomprises positioning a substrate within a processing chamber andheating the substrate to a temperature within a range from about 600degrees Celsius to about 650 degrees Celsius. One or more process gasesare then introduced into the processing chamber. The one or more processgases comprise a silicon source and a phosphorus source. A substantiallycarbon-free silicon epitaxial layer is then deposited on the substrate.The substantially carbon-free silicon epitaxial layer has a phosphorusconcentration of about 1×10²¹ atoms per cubic centimeter or greater andis deposited at a chamber pressure of about 300 Torr or greater.

In another embodiment, a method of forming a film on a substratecomprises positioning a substrate within a processing chamber andheating the substrate to a temperature within a range from about 550degrees Celsius to about 750 degrees Celsius. Phosphine and at least onesilane or disilane are then introduced into the processing chamber and asubstantially carbon-free silicon epitaxial layer is deposited on thesubstrate. The substantially carbon-free silicon epitaxial layer has aphosphorus concentration of about 1×10²¹ atoms per cubic centimeter orgreater and is deposited at a chamber pressure of about 150 Torr orgreater.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a flow chart illustrating a method of forming aphosphorus-containing silicon epitaxial layer.

FIG. 2 is a graph illustrating the dopant profile of a film formedaccording to embodiments of the invention.

FIG. 3 is a graph illustrating the tensile stress of the film of FIG. 2.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

Embodiments of the present invention generally relate to methods forforming silicon epitaxial layers on semiconductor devices. The methodsinclude forming a silicon epitaxial layer on a substrate at increasedpressure and reduced temperature. The silicon epitaxial layer has aphosphorus concentration of about 1×10²¹ atoms per cubic centimeter orgreater, and is formed without the addition of carbon. A phosphorusconcentration of about 1×10²¹ atoms per cubic centimeter or greaterincreases the tensile strain of the deposited layer, and thus, improveschannel mobility. Since the epitaxial layer is substantially free ofcarbon, the epitaxial layer does not suffer from film formation andquality issues commonly associated with carbon-containing epitaxiallayers. Substantially free of carbon as used herein refers to a filmwhich is formed without the use of a carbon-containing precursor;however, it is contemplated that trace amounts of carbon may be presentin the film due to contamination.

Embodiments of the present invention may be practiced in the CENTURA® RPEpi chamber available from Applied Materials, Inc., of Santa Clara,Calif. It is contemplated that other chambers, including those availablefrom other manufacturers, may be used to practice embodiments of theinvention.

FIG. 1 is a flow chart 100 illustrating a method of forming aphosphorus-containing silicon epitaxial layer. In step 102, amonocrystalline silicon substrate is positioned within a processingchamber. In step 104, the substrate is heated to a predeterminedtemperature. The substrate is generally heated to a temperature within arange from about 550 degrees Celsius to about 700 degrees Celsius. It isdesirable to minimize the thermal budget of the final device by heatingthe substrate to the lowest temperature sufficient to thermallydecompose process reagents and deposit an epitaxial film on thesubstrate. However, as increased temperatures generally lead toincreased throughput, it is contemplated that higher temperatures may beused as dictated by production requirements.

In step 106, process gases containing one or more processing reagentsare introduced into the processing chamber. The process gases include asilicon source and phosphorus source for depositing aphosphorus-containing silicon epitaxial layer on the substrate.Optionally, the one or more process gases may include a carrier gas fordelivering the silicon source and the phosphorus source to theprocessing chamber, as well as an etchant when performing selectivedeposition processes.

An exemplary phosphorus source includes phosphine, which may bedelivered to the processing chamber at a rate of about 2 sccm to about30 sccm or greater. For example, the flow rate of phosphine may be about12 sccm to about 15 sccm. Suitable carrier gases include nitrogen,hydrogen, or other gases which are inert with respect to the depositionprocess. The carrier gas may be provided to the processing chamber at aflow rate within a range from about 3 SLM to about 30 SLM. Suitablesilicon sources include dichlorosilane, silane, and disilane. Thesilicon source may be delivered to the processing chamber at a flow ratebetween about 300 sccm and 400 sccm. While other silicon and phosphorussources are contemplated, it is generally desirable that carbon additionto the processing atmosphere is minimized, thus, carbon-containingprecursors should be avoided.

In step 108, the mixture of reagents is thermally driven to react anddeposit a phosphorus-containing silicon epitaxial layer on the substratesurface. During the deposition process, the pressure within theprocessing chamber is maintained at about 150 Torr or greater, forexample, about 300 Torr to about 600 Torr. It is contemplated thatpressures greater than about 600 Torr may be utilized when low pressuredeposition chambers are not employed. In contrast, typical epitaxialgrowth processes in low pressure deposition chambers maintain aprocessing pressure of about 10 Torr to about 100 Torr and a processingtemperature greater than 700 degrees Celsius. However, by increasing thepressure to about 150 Torr or greater, the deposited epitaxial film isformed having a greater phosphorus concentration (e.g., about 1×10²¹atoms per cubic centimeter to about 5×10²¹ atoms per cubic centimeter)compared to lower pressure epitaxial growth processes. Furthermore, highflow rates of phosphorus source gas provided during low pressuredepositions often result in “surface poisoning” of the substrate, whichsuppresses epitaxial formation. Surface poisoning is typically notexperienced when processing at pressures above 300 Torr, due to thesilicon source flux overcoming the poisoning effect. Thus, increasedprocessing pressures are desirable for epitaxial processes utilizinghigh dopant flow rates.

The phosphorus concentration of an epitaxial film formed at a pressureless than 100 Torr is approximately 3×10²⁰ atoms per cubic centimeterwhen providing a phosphine flow rate of about 3 sccm to about 5 sccm.Thus, epitaxial layers formed at higher pressures (e.g., 300 Torr orgreater) experience approximately a tenfold increase in phosphorusconcentration compared to epitaxial films formed at pressures belowabout 100 Torr or less. It is believed that at a phosphorusconcentration of about 1×10²¹ atoms per cubic centimeter or greater, thedeposited epitaxial film is not purely a silicon film doped withphosphorus, but rather, that the film is an alloy between silicon andsilicon phosphide (e.g., pseudocubic Si₃P₄). It is believed that thesilicon/silicon phosphide alloy attributes to the increased tensilestress of the epitaxial film. The likelihood of forming thesilicon/silicon phosphide alloy increases with greater phosphorusconcentrations, since the probability of adjacent phosphorus atomsinteracting is increased.

Epitaxial films which are formed at process temperatures between about550 degrees Celsius and about 750 degrees Celsius and at pressures above300 Torr experience increased tensile stress when doped to a sufficientphosphorus concentration (e.g., about 1×10²¹ atoms per cubic centimeteror greater). Carbon-free epitaxial films formed under such conditionsexperience approximately 1 gigapascal to about 1.5 gigapascals oftensile stress, which is equivalent to a low pressure silicon epitaxialfilm containing about 1.5 percent carbon. However, as noted above,epitaxial films containing greater than about 1 percent carbon sufferfrom decreased film quality, and are thus undesirable. Furthermore,carbon-doped silicon epitaxy processes typically utilize cyclicaldeposition-etch processes which increase process complexity and cost.Producing an epitaxial film according to embodiments herein not onlyresults in a film having a tensile stress equal to or greater than a 1.5percent carbon-containing epitaxial film, but the resistivity of thecarbon-free film is also lower (e.g., about 0.6 milliohm-centimeterscompared to about 0.9 milliohm-centimeters). Thus, the substantiallycarbon-free epitaxial film exhibits higher film quality, lowerresistivity, and equivalent tensile stress when compared tocarbon-containing epitaxial films.

The tensile strain of the epitaxially-grown film can further beincreased by reducing the deposition temperature during the epitaxialgrowth process. In a first example, a phosphorus-doped silicon epitaxialfilm is deposited at a chamber pressure of 700 Torr and a temperature ofabout 750 degrees Celsius. Process gases containing 300 sccm ofdichlorosilane and 5 sccm of phosphine were provided to a processchamber during the growth process. The deposited film contained aphosphorus concentration of about 3×10²⁰ atoms per cubic centimeter, andexhibited a tensile strain equal to a silicon epitaxial film having acarbon concentration of about 0.5 atomic percent. In a second example, aphosphorus-doped silicon epitaxial film was deposited on anothersubstrate under similar process conditions; however, the processtemperature was reduced to about 650 degrees Celsius, and the flow rateof phosphine was increased to 20 sccm. The phosphorus-doped siliconepitaxial film had a tensile strain equivalent to a film containing 1.8atomic percent carbon. Thus, as process temperature is reduced anddopant concentration is increased, the tensile strain within thedeposited epitaxial film increases. It is to be noted, however, that thetensile strain benefits due to decreased temperature may be limited,since there is minimum temperature which is required to react anddeposit the process reagents.

In a third example, a phosphorus-doped silicon epitaxial film was formedunder similar process conditions as the first example; however, the flowrate of phosphine during processing was reduced to about 2 sccm. Theresultant phosphorus-doped silicon epitaxial film had a tensile strainequivalent to a film having about 0.2 percent carbon. Additionally, theresultant film had a resistivity of about 0.45 milliohm-centimeterscompared to 0.60 milliohm-centimeters for the film of the first example.Thus, not only can the tensile strain of an epitaxial film be adjustedby varying temperature and or pressure during the deposition process,but the resistivity can also be adjusted by varying the amount of dopantprovided to the processing chamber.

FIG. 2 is a graph illustrating the dopant profile of a film formedaccording to embodiments of the invention. The analyzed film of FIG. 2was formed by heating a silicon substrate having a silicon-germaniumlayer thereon to a temperature of about 650 degrees Celsius.Approximately 300 sccm of dichlorosilane and 30 sccm of phosphine weredelivered to a processing chamber maintained at a pressure of about 600Torr. A 450 angstrom silicon epitaxial film was formed on thesilicon-germanium layer. As determined by secondary ion massspectroscopy, the phosphorus-doped epitaxial film had a uniformphosphorus concentration of about 3×10²¹ atoms per cubic centimeter, andwas substantially free of carbon. In contrast to the film analyzed inFIG. 2, epitaxial films formed at lower pressures, such as less than 300Torr, have a phosphorus concentration of about 3×10²⁰ atoms per cubiccentimeter. Thus, the epitaxial film formed according to embodimentsdescribed herein exhibited a tenfold increase in phosphorusconcentration as compared to epitaxial films formed at lower pressures.

FIG. 3 is a graph illustrating the tensile stress of the film of FIG. 2as determined by high resolution X-ray diffraction. The peak Acorresponds to the tensile stress of the monocrystalline siliconsubstrate, while the peak B corresponds to the tensile stress of thesilicon-germanium layer. The peak C corresponds to the tensile stress ofthe phosphorus-containing epitaxial layer. The well defined edges of thepeak B and the peak C are indicative of high quality epitaxial filmshaving uniform composition. The peak B corresponds to asilicon-germanium epitaxial layer containing about 12.3 percentgermanium. The peak B has a shift between about −1000 arc seconds andabout −1500 arc seconds (e.g., compressed stress), and an intensity ofabout 1000 a.u. The peak C has a peak shift of about 1700 arc seconds toabout 2400 arc seconds (e.g., tensile stress), and an intensity of about800 a.u. The stress corresponding to peak C is similar to that of anepitaxial film having a carbon concentration of about 1.8 atomicpercent. As discussed above, epitaxial films containing greater thanabout 1 atomic percent carbon have unacceptable film quality. Thus,while the tensile strength of highly phosphorus-doped epitaxial films isabout equal to an epitaxial film containing 1.8 atomic percent carbon,the highly phosphorus-doped epitaxial films exhibit a higher filmquality than the carbon-doped epitaxial films of comparable tensilestrain.

Benefits of the invention include high quality silicon epitaxial filmsexhibiting high tensile strain. Increased process pressures combinedwith reduced process temperatures allow for formation of a siliconepitaxial film having a phosphorus concentration of 3×10²¹ atoms percubic centimeter or greater, without experiencing surface poisoning. Thehigh phosphorus concentration induces stress within the depositedepitaxial film, thereby increasing tensile strain, leading to increasedcarrier mobility and improved device performance. The tensile strainobtained by highly phosphorus-doped epitaxial silicon is comparable toepitaxial films containing up to 1.8 atomic percent carbon. However,highly phosphorus-doped epitaxial silicon of the present inventionavoids the quality issues associated with carbon-doped films.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

We claim:
 1. A method of forming a film on a substrate, comprising: positioning a substrate within a processing chamber; heating the substrate to a temperature within a range from about 550 degrees Celsius to about 750 degrees Celsius; introducing one or more process gases into the processing chamber, the one or more process gases comprising a silicon source and a phosphorus source; and depositing a substantially carbon-free epitaxial layer comprising Si₃P₄ on the substrate, the substantially carbon-free epitaxial layer having a phosphorus concentration of 1×10²¹ atoms per cubic centimeter or greater, wherein the substantially carbon-free epitaxial layer is deposited at a chamber pressure of about 150 Torr or greater.
 2. The method of claim 1, wherein the chamber pressure is about 300 Torr or greater.
 3. The method of claim 1, wherein the silicon precursor is dichlorosilane.
 4. The method of claim 3, wherein the phosphorus precursor is phosphine.
 5. The method of claim 1, wherein the temperature is within a range from about 600 degrees Celsius to about 650 degrees Celsius.
 6. The method of claim 5, wherein the silicon precursor is silane or disilane.
 7. The method of claim 1, wherein the substantially carbon-free epitaxial layer has a tensile strain of about 1 gigapascal to about 1.5 gigapascals.
 8. A method of forming a film on a substrate, comprising: positioning a substrate within a processing chamber; heating the substrate to a temperature within a range from about 600 degrees Celsius to about 650 degrees Celsius; introducing one or more process gases into the processing chamber, the one or more process gases comprising a silicon source and a phosphorus source; and depositing a substantially carbon-free epitaxial layer comprising Si₃P₄ on the substrate, the substantially carbon-free epitaxial layer having a phosphorus concentration of 1×10²¹ atoms per cubic centimeter or greater, wherein the substantially carbon-free epitaxial layer is deposited at a chamber pressure of about 300 Torr or greater.
 9. The method of claim 8, wherein the substantially carbon-free epitaxial layer has a tensile strain of about 1 gigapascal to about 1.5 gigapascals.
 10. The method of claim 9, wherein the silicon precursor is silane or disilane.
 11. The method of claim 8, wherein the phosphorus precursor is phosphine.
 12. The method of claim 11, wherein the silicon precursor is dichlorosilane.
 13. A method of forming a film on a substrate, comprising: positioning a substrate within a processing chamber; heating the substrate to a temperature within a range from about 550 degrees Celsius to about 750 degrees Celsius; introducing phosphine and at least one of silane or disilane into the processing chamber; and depositing a substantially carbon-free epitaxial layer comprising Si₃P₄ on the substrate, the substantially carbon-free epitaxial layer having a phosphorus concentration of 1×10²¹ atoms per cubic centimeter or greater, wherein the substantially carbon-free epitaxial layer is deposited at a chamber pressure of about 150 Torr or greater.
 14. The method of claim 13, wherein the chamber pressure is about 300 Torr or greater.
 15. The method of claim 14, wherein the temperature is within a range from about 600 degrees Celsius to about 650 degrees Celsius.
 16. The method of claim 15, wherein the substantially carbon-free epitaxial layer has a tensile strain of about 1 gigapascal to about 1.5 gigapascals. 